This application claims priority under 35 U.S.C § 119(a) on patent application No. 2003-397479 filed in Japan on Nov. 27, 2003, the entire contents of which are hereby incorporated by reference.
The present invention relates to an optical semiconductor element in which a light-emitting element and a light-receiving element are placed directly opposite to each other. The present invention relates especially to optical semiconductor elements and electronic devices using such optical semiconductor elements that require an improvement of resistance against noise from electromagnetic waves from circuits within the electronic devices as well as electromagnetic waves from outside the electronic devices.
As mobile phones, wireless LANs that comply with the wireless LAN (Local Area Network) standard IEEE (Institute of Electrical & Electronics Engineers) 802.11a/B/G, for example, and Bluetooth come into wide use, there is an increasing need for measures against incident electromagnetic waves from outside the electronic devices. Especially, the frequency of the electromagnetic waves ranges from several hundred megahertz (MHz) to several gigahertz (GHz), which is higher than radio and television waves, and because their energy is also high and their wavelength is short, electromagnetic leakage easily occurs. Therefore, utmost caution is called for in taking measures for electromagnetic interference shielding in environment where the devices are close to each other.
FIG. 15 is a cross-sectional view showing an example of a conventional optical semiconductor element.
This optical semiconductor element includes a light-emitting element 101, a light-receiving element 102 that is placed directly opposite to the light-emitting element 101, lead frames 103a and 103b on which the light-emitting element 101 and the light-receiving element 102 are respectively die bonded, a transparent resin portion 104 that covers those portion of the light-emitting element 101, the light-receiving element 102, and the lead frames 103a and 103b where the light-emitting element 101 as well as the light-receiving element 102 are die bonded, and a light-blocking resin portion 105 that covers the surface of the transparent resin portion 104.
In the optical semiconductor element, because the shape of the lead frames 103a and 103b is flat, incident electromagnetic waves from outside, especially from the side, easily reach the light-receiving element 102 and may lead to malfunctioning of the light-receiving element.
In such conventional optical semiconductor elements and the substrates on which the optical semiconductor elements are mounted, various measures have been taken to prevent malfunctions due to electric fields arising from electronic devices or wirings that are close to each other.
For example, as an example of measures taken for the light-receiving element, one measure is that of using a metal shielding wire as a wiring to connect an emitter electrode and another measure is that of forming a transparent conductive film on the emitter electrode installed above the light-receiving element portion in order to protect especially the periphery of a base electrode as well as a base layer region. Source of noise in this case is a rapid displacement of the electric field, and intrusion of electromagnetic waves is prevented by means of dispersing the potential lines across the base portion which includes the base layer and the base electrode on the light-receiving element to the metal shielding wire and to liberate the electric, charge that is subsequently generated inside the metal shielding wire to outside the device.
On the other hand, as an example of measures taken on the exterior of electronic devices in which optical semiconductor elements are mounted, one measure is to prevent release as well as intrusion of electromagnetic waves from outside by completely covering the electronic device with a metal case and the like. Source of noise in this case is electromagnetic waves approaching from far away as a plane wave and the intrusion and release of the electromagnetic wave are prevented by means of taking advantage of the phenomenon that electromagnetic waves absorbed in metal plates are converted to heat inside the metal plates thus blocking the intrusion of electromagnetic waves to electronic parts and optical semiconductor elements.
Generally, the shield effect of metallic lead frames used for conventional electronic devices is such that the skin depth δ for a frame made of copper at high frequencies (of 1 GHz) as are emitted by mobile phones or the like is 2.09 μm. It should be noted that the skin depth δ is given by δ=2/√(2πfμσ) where f denotes frequency, μ denotes magnetic permeability, and σ denotes conductivity. Therefore, if the thickness of the copper frame is about the same as the thickness of the skin depth, then it is possible to attenuate the electromagnetic waves penetrating the metal plates when electromagnetic waves are irradiated onto the metal plates to 1/e (about 37%) in intensity and 1/e2 (about 13.5%) in power through the attenuation inside the copper frame, provided that the reflection of the electromagnetic wave at the surface is ignored, where e is the energy of the irradiated wave.
Furthermore, if the thickness of the metal plate is 3 times thicker than the skin depth, then it is possible to attenuate the irradiated electromagnetic wave to about 1/20 (−26 dB), and if the thickness of the metal plate is 5 times thicker than the skin depth, then it is possible to attenuate the irradiated electromagnetic wave to about 1/150 (−43 dB). More than enough shield effect is obtained with the thickness of a common lead frame (100 to 200 μm).
Therefore, what is important in protecting against high frequency electromagnetic waves is not the thickness of the shielding, but that the protection is a package equipped with shielding that can protect against electromagnetic waves irradiated from all directions.
FIG. 16 is a cross-sectional view showing an example of a conventional optical semiconductor element equipped with an oxide insulating film as a shield against electromagnetic waves.
The optical semiconductor element is equipped with an oxide insulating film 106 as a metal shield plate on the surface of a light-blocking resin portion 105 and the internal elements are protected by the oxide insulating film 106. However, provided with this structure, electrical insulation between the light-receiving element and the light-emitting element is not possible, and the fundamental ability of the photocoupler, which is to provide electrical insulation between the light-receiving element and the light-emitting element, is lost.
As a conventional structure and method for attenuation of electromagnetic radiation, JP H7-7099A discloses an integrated circuit package as well as a method for attenuating electromagnetic radiation. However, in optical couplers such as photocouplers for which the optical coupler disclosed in JP S62-247575A is a typical example, nothing in particular is mentioned about electromagnetic interference shielding and since the light-emitting element and the light-receiving element are electrically insulated, it is impossible to completely cover the components by metal plates.
In the above-described photocoupler, since no measure is taken against the electromagnetic waves incident from outside, there is the possibility of malfunctioning of the light-receiving element inside.
If the entire component is covered by metal plates, then there is no electrical insulation between the light-receiving element and the light-emitting element, and the basic functionality of the photocoupler is lost. Therefore, the light-emitting element and the light-receiving element must individually be covered by shields. As such a shielding method, it is conceivable to place a shield made of metal or metallic mesh between the light-receiving element and the light-emitting element. But in this case, the optical transmission path is blocked and the transmission efficiency considerably deteriorates.
Therefore, an important point to keep in mind when taking measures against electromagnetic waves incident from outside is how easily loss can be caused by the shield. The loss is divided into three kinds: reflection loss, absorption loss, and loss due to multiple reflection. Reflection loss is the loss at the interface between the surface of the shield and the external medium; it is not affected by the thickness of the shield, and is dependent on the conductivity of the shield. Also, absorption loss is the loss generated by an excess current which in turn is generated as electromagnetic waves pass through the shield; the absorption loss is affected by the thickness of the shield and the material of the shield (magnetic permeability and conductivity). Furthermore, loss due to multiple reflections is the loss that occurs when the electromagnetic waves are repeatedly reflected inside the shield material.
Therefore, if the object is to absorb the electromagnetic waves, then it is preferable that the chip is surrounded in all directions with metal plates with high conductivity such as copper, that reflection loss, absorption loss, and loss through multiple reflections are utilized to the extent possible, and to ensure that there is no path that lets electromagnetic waves reach the light-receiving element.
Since the structure of the lead frames of the conventional photocoupler has many crevices that allow electromagnetic waves to enter, there is the problem that it is not possible to simultaneously achieve an increase in the resistance to noise from electromagnetic waves and to achieve electrical insulation between the light-receiving element and the light-emitting element as well as a high light reception efficiency.
It is thus an object of the present invention to provide an optical semiconductor element equipped with a package that has high absorptive power for external electromagnetic waves as well as an electronic device using this optical semiconductor element by improving the shape of a header of a lead frame on the side of the light-receiving element and by measures in the assembly process.